The present invention relates to a position detecting apparatus used for machine tools, such as milling machines, for detecting the position of a reference point on a workpiece.
In the prior art, a reference point for the start of the cutting operation is set on the workpiece and a predetermined cutting pattern is begun from this reference point. A position detecting apparatus is used for setting this reference point. Such a position detecting apparatus is equipped with a conductive sensor 10, which generates a position detection signal when conductive sensor 10 comes into contact with the milling surface of the workpiece, as shown in FIG. 1A.
In order to set the milling reference position using this position detecting apparatus, the apparatus is attached to the cutter shaft of the milling machine. The reference surface of the prepositioned workpiece 11 is placed in contact with sensor 10 of the position detecting apparatus and a position detection signal is output to set the reference scale for the milling machine at zero. After the reference position for the start of the cutting operation is set in this way, the detecting apparatus is removed and the cutter is replaced on the shaft, and milling is begun.
As shown in FIG. 1A, sensor 10 of the position detecting apparatus is vertically supported in relation to milling table 12 by spring 13. Spring 13 is located in the hollow space of metal cylinder 14 which is the support for sensor 10. As shown in FIG. 1B, sensor 10 is able to move slightly horizontally when coming into contact with the workpiece to thereby reduce the shock.
However, sensor 10 receives a shock not only in the horizontal direction, but in the vertical direction as well. The contact portion of sensor 10 has a structure such as that shown in FIG. 1A, and, when workpiece 11 is moved a little too far from the original position, it is very difficult to absorb the shock to sensor 10 in the horizontal direction. In the prior art, this situation often resulted in the position detecting apparatus being damaged by the shock of contact with the workpiece.